Résumé : Maintaining crop production over time while using different cultural practices was studied in the Center North zone of Burkina Faso in two trials where the evolution of groundnut and sorghum yields in rotation was observed for 8 years. The trials compared 6 types of cultural practices (manual tillage, plowing, plowing + compost supply, associated or not with mineral fertilization). The plots in which these cultural practices were applied were split into 4 subplots which were sown successively for 4 years (Table 2) after fallow. Annual measurements were made of yield gap resulting from the differing numbers of years of cultivation. One of the trials was set up on a gravel soil with a shallow hardpan, the other on a deeper sandy loam soil. The effects of techniques and of cultivation were observed on some variables representative of the different stages which lead to final production of seeds (number of pods, percentage of two-seeded pods, average weight of one seed for groundnut - number of stems, number of ears per stem, number of seeds per ear and average weight of one seed for sorghum).
Results show that cultivating reduces groundnut and sorghum yields (Tables 4, 7 and 8). Groundnut yield variations are caused mainly by variations in pod number (Figure 1), which is linked to the vegetative growth of the plant. Overall, the percentage of two-seeded pods and the weight of a hundred seeds are affected mainly by cultivation (Table 6). In addition, the number of years of cultivation affects the rate of damaged pods on gravel soils. Mineral fertilization partly offsets these effects, but it does not maintain the same ratio of pod weight to vegetative biomass as in the initial conditions. The contrast in the effect on this ratio of mineral fertilization and compost supplies during the last years of cultivation shows the essential function of these supplies.
Generally, the number of years of cultivation affects the number of sorghum seeds and their average mean weight negatively (Table 9). The result is a decrease in the ratio of seed weight to straw weight. On gravel soils, lack of fertilization on soil disrupts ear setting (Figure 4). The decrease in production in plots without organic or mineral fertilizers is increasingly apparent in comparison to fertilized plots, with a main effect on seed filling and on the ratio of seed weight to straw weight (Table 9). On sandy loam soils, manual tillage also speeds up the decrease in seed yield. Plowing mainly affects crop installation and the number of stems and, thus, appears to be a limiting factor only for high yield (Figure 3).
Cultivating alters the chemical characteristics of the soil (decrease in the level of organic matter, in pH, in total exchangeable bases - Table 10). Less intensive cultural practices, without organic and mineral restitution, produce the largest N and P deficits (Table 11) while inducing smaller K, Ca and Mg losses than mineral fertilization. Compost supplies correct the balance for Ca and Mg (Table 11) but cannot maintain the stock of organic matter in the soil. Mineral fertilization improves the phosphorus balance (Table 11) but has adverse effects on the calcium balance as it increases exports without compensation. Mineral fertilization reduces pH (Table 10). The response to cultural practices, hence their profitability, is comparable for both soils (Table 12).
In conclusion, the lack of organic and mineral fertilization tends to increase the negative effect of the number of years of cultivation and appears to be environmentally destructive. Intensification is a necessity, nutritional concerns aside, to maintain soil fertility. Current farmer strategies take into account the specificity of the crops by cultivating sorghum during the first years after fallow, then continuing with groundnut which makes a better use of exhausted soil because of its favorable ratio of marketable weight to total biomass. This approach represents a mining strategy of soil use, aimed at total use followed by field abandonment. The production costs of intensification are high and should be supported by an efficient policy of fertilizer distribution.